The migration of vascular smooth muscle cells (VSMCs) is a key event in the pathogenesis of many vascular diseases. We have previously shown that VSMC migration in response to PDGF is suppressed in differentiated VSMCs and hav sought to identify differences in intracellular signalling between differentiated and proliferating VSMCs that may account for this suppression. Differentiated VSMCs retain their ability to respond to PDGF and upregulate expression of the immediate early response genes, c-fos and MCP-1 (JE) when stimulated by PDGF. Unlike proliferating cells, however, PDGF-stimulated differentiated VSMCs fail to activate calcium/calmodulin-dependent protein kinase (CamKinase) II activity. Blocking CamKinase II activation blocked the migration of proliferating VSMCs by more than 90%. In contrast, inhibitors of protein kinase C had no significant effect on migration. Pretreatment of differentiated cells with ionomycin (1 uM) resulted in an 84 + 6% return to the migration rate of proliferating VSMCs. This return was also blocked by CamKinase inhibitors and was unaffected by inhibitors of PKC. These results suggest that activation of CamKinase plays an important role in VSMC migration and the failure to activate it in differentiated VSMCs may be responsible for the suppression of migration. In addition to PDGF, migration also requires the autocrine production of the growth factor, basic FGF. bFGF blocking antibodies inhibit the PDGF-directed migration of proliferating cells, whil the addition of exogenous bFGF enables growth-arrested cells to migrate in response to PDGF. bFGF's effect on migration is likely due its role in enabling PDGF to activate CamKII since 1) bFGF antibodies block activation of CamKII in response to PDGF, while the migration of VSMCs expressing constitutively active CamKII is not affected by these antibodies; and 2) th ability of exogenous bFGF to stimulate migration in growth-arrested cells i blocked by CamKII inhibition. These results demonstrate that, while multiple intracellular signalling pathways triggered by chemoattractant recognition may be required for migration, the regulation VSMCs is controlled by CamKII activation. In addition, these results also demonstrate that to activate VSMC migration requires concurrent action by at least two growth factors/cytokines. This requirement may limit the response of VSMCs to injury to selected group capable of responding to agents and provides a molecular basis for investigating the different abilities of various VSMC populations to migrate.